Conductors and Insulators

After reading this section you will be able to do the following:

  • Contrast the characteristics of conductors and insulators.
  • List examples of common conductors and insulators.
  • Explain how insulators provide protection from electricity.
  • Describe what a dielectric and dielectric properties are.

In the previous pages, we have talked a bit about “conductors” and “insulators”. We will discuss these two subjects a little more before moving on to discuss circuits.

Conductors

Copper is considered to be a conductor because it “conducts” the electron current or flow of electrons fairly easily. Most metals are considered to be good conductors of electrical current. Copper is just one of the more popular materials that is used for conductors.

Other materials that are sometimes used as conductors are silver, gold, and aluminum. Copper is still the most popular material used for wires because it is a very good conductor of electrical current and it is fairly inexpensive when compared to gold and silver. Aluminum and most other metals do not conduct electricity quite as good as copper.

Some examples of things made out of conductors are shown below.

Insulators

Insulators are materials that have just the opposite effect on the flow of electrons that conductors do. They do not let electrons flow very easily from one atom to another. Insulators are materials whose atoms have tightly bound electrons. These electrons are not free to roam around and be shared by neighboring atoms.

Some common insulator materials are glass, plastic, rubber, air, and wood.

Insulators are used to protect us from the dangerous effects of electricity flowing through conductors. Sometimes the voltage in an electrical circuit can be quite high and dangerous. If the voltage is high enough, electric current can be made to flow through even materials that are generally not considered to be good conductors. Our bodies will conduct electricity and you may have experienced this when you received an electrical shock. Generally, electricity flowing through the body is not pleasant and can cause injuries. The function of our heart can be disrupted by a strong electrical shock and the current can cause burns. Therefore, we need to shield our bodies from the conductors that carry electricity. The rubbery coating on wires is an insulating material that shields us from the conductor inside. Look at any lamp cord and you will see the insulator. If you see the conductor, it is probably time to replace the cord.

Recall our earlier discussion about resistance. Conductors have a very low resistance to electrical current while insulators have a very high resistance to electrical current. These two factors become very important when we start to deal with actual electrical circuits.

Dielectrics and Dielectric Properties

Some electrical insulators can be polarized by an electric field. These materials are called dielectrics. Dielectrics can be characterized by their dielectric properties which are quantified by a complex dielectric constant/complex permittivity. These two quantities are the same and are often used interchangably. The complex dielectric constant consists of two parts: the permittivity and the loss factor. The permittivity is the real part of the complex number that makes up the complex dielectric constant and the loss factor is the imaginary part of the complex number. The complex dielectric constant is represented by the equation below:

ϵ r = ϵ r j ϵ r \epsilon_r={\epsilon_r^{'}-j\epsilon_r^{''}}

ϵ r : p e r m i t t i v i t y {\epsilon_r^{'}: permittivity}

ϵ r : l o s s f a c t o r {\epsilon_r^{''}: loss factor}

At microwave frequencies, the permittivity describes how well a material stores microwave energy, while the loss factor describes how well a material absorbs microwave energy.

In the equations above, you'll also notice the subscript r's. These represent that this is the relative dielectric constant. It is called the relative dielectric constant because it's the value relative to the permittivity of free space which is 8.854x10^-12 F/m. Dielectric properties can be thought of the electric version of permeability.

Review

  1. Conductors conduct electrical current very easily because of their free electrons.
  2. Insulators oppose electrical current and make poor conductors.
  3. Some common conductors are copper, aluminum, gold, and silver.
  4. Some common insulators are glass, air, plastic, rubber, and wood.
  5. Insulators that can be polarized by an electric field are called dielectrics.